linux-kernel - Re: [PATCH v5 5/5] intel

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Date:   Thu, 13 Jul 2017 07:11:22 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "dbasehore ." <dbasehore@...omium.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Rajneesh Bhardwaj <rajneesh.bhardwaj@...el.com>,
        x86@...nel.org,
        Platform Driver <platform-driver-x86@...r.kernel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>,
        Linux-pm mailing list <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v5 5/5] intel_idle: Add S0ix validation

On Wed, 12 Jul 2017, dbasehore . wrote:
> On Wed, Jul 12, 2017 at 3:16 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > There are more issues with this: If there is a hrtimer scheduled on that
> > last CPU which enters the idle freeze state and that timer is 10 minutes
> > away, then the check timer can't be programmed and the system will happily
> > stay for 10 minutes in some shallow C state without notice. Not really
> > useful.
> 
> Are hrtimers not suspended after timekeeping_suspend is called?

They are. As I said I forgot about the inner workings and that check for
state != shutdown confused me even more, as it just looked like this might
be a valid state.

> > You know upfront whether the i915 power wells (or whatever other machinery)
> > is not powered off to allow the system to enter a specific power state. If
> > you think hard enough about creating infrastructure which allows you to
> > register power related facilities and then check them in that idle freeze
> > enter state, then you get immediate information WHY this happens and not
> > just the by chance notification about the fact that it happened.
> 
> It's not always something that can be checked by software. There was
> one case where an ordering for powering down audio hardware prevented
> proper PC10 entry, but there didn't seem to be any way to check that.
> Hardware watchdogs also have the same lack of clarity, but most if not
> all desktop and mobile processors ship with one. Overall, this seems
> to be the best that can be done at this point in freeze, and we can't
> really rely on every part of the system properly validating it's state
> in its suspend operation.

So if I understand correctly, this is the last resort of catching problems
which can't be detected upfront or are caused by a software bug.

I'm fine with that, but please explain and document it proper. The current
explanation is confusing at best.

Thanks,

	tglx